--- a/m209/keylist/config.py	Wed Jun 05 21:23:30 2013 -0500
+++ b/m209/keylist/config.py	Thu Jun 06 20:32:37 2013 -0500
@@ -38,15 +38,15 @@
 WHEELS = ['wheel{}'.format(n) for n in range(1, 7)]
 
 
-def read_key_list(fp, indicator):
-    """Reads from fp, which must be an iterable that yields unicode strings.
+def read_key_list(fname, indicator):
+    """Reads key list information from the file given by fname.
 
     Searches the config file for the key list with the given indicator. If
     found, returns a KeyList object. Returns None if not found.
 
     """
     config = configparser.ConfigParser(interpolation=None)
-    config.read_file(fp)
+    config.read(fname)
 
     if indicator not in config.sections():
         return None
@@ -59,5 +59,31 @@
             letter_check=section['check'])
 
 
-def write_key_list(fp, key_list):
-    pass
+def write_key_list(fname, key_list):
+    """Updates the file named by fname with the given key_list.
+
+    If the file doesn't exist, it is created and the key_list is written to it.
+
+    If the file already exists, it is read and searched for a section that
+    matches the key_list indicator name. If found, this section is updated and
+    the file is written back out. If the section is not found, one for the
+    key_list is added and the file is written out.
+
+    """
+    config = configparser.ConfigParser(interpolation=None)
+    config.read(fname)
+
+    # If the section for this key list doesn't exist, add one
+    if not config.has_section(key_list.indicator):
+        config.add_section(key_list.indicator)
+
+    # Now update it
+    section = config[key_list.indicator]
+    section['lugs'] = key_list.lugs
+    for n, wheel in enumerate(WHEELS):
+        section[wheel] = key_list.pin_list[n]
+    section['check'] = key_list.letter_check
+
+    # Write the file
+    with open(fname, 'w') as fp:
+        config.write(fp)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/m209/keylist/tests/test_config.py	Thu Jun 06 20:32:37 2013 -0500
@@ -0,0 +1,58 @@
+# Copyright (C) 2013 by Brian Neal.
+# This file is part of m209, the M-209 simulation.
+# m209 is released under the MIT License (see LICENSE.txt).
+
+"""Unit tests for the key list read/write routines for the config file
+format.
+
+"""
+
+from contextlib import contextmanager
+import os
+import tempfile
+import unittest
+
+from ..key_list import KeyList
+from ..config import read_key_list, write_key_list
+
+
+@contextmanager
+def file_remover(path):
+    """Ensures a file is deleted."""
+    try:
+        yield
+    except Exception:
+        raise
+    finally:
+        os.remove(path)
+
+
+class ConfigFileTestCase(unittest.TestCase):
+
+    def test_round_trip(self):
+
+        key_list1 = KeyList(
+                indicator='AA',
+                lugs='0-4 0-5*4 0-6*6 1-0*5 1-2 1-5*4 3-0*3 3-4 3-6 5-6',
+                pin_list= [
+                    'FGIKOPRSUVWYZ',
+                    'DFGKLMOTUY',
+                    'ADEFGIORTUVX',
+                    'ACFGHILMRSU',
+                    'BCDEFJKLPS',
+                    'EFGHIJLMNP'
+                ],
+                letter_check='QLRRN TPTFU TRPTN MWQTV JLIJE J')
+
+        fd, path = tempfile.mkstemp(suffix='.ini', text=True)
+        os.close(fd)
+
+        with file_remover(path):
+            write_key_list(path, key_list1)
+            key_list2 = read_key_list(path, key_list1.indicator)
+
+            self.assertEqual(key_list1, key_list2)
+
+            kl3 = read_key_list(path, 'BB')
+            self.assertTrue(kl3 is None)
+